The present invention relates to passive cooling of a nuclear reactor containment. The invention is particularly directed to an isolation condenser type passive cooling system which can be installed externally of the nuclear reactor containment as well as building structure in which the containment is situated, the cooling system being such as minimizes need to penetrate the containment and building enclosure with cooling system components.
U.S. Pat. Nos. 5,059,385, 5,082,619 and 5,106,571 disclose use of isolation condensers in connection with passive removal of initial and decay heat loads generated in a nuclear reactor system containment as a result of and upon occurrence of a LOCA, i.e., a loss-of-coolant accident in the system.
The cooling systems disclosed in these pending applications also can dissipate initial heat by venting the reactor pressure vessel and/or the containment drywell space to a suppression pool of water confined in a chamber surrounding the reactor pressure vessel. Venting to the suppression pool also can be used with respect to condensate recovery of the isolation condensers, and non-condensable gasses such as nitrogen, which are cooled in an isolation condenser and separated from the condensate.
Venting from the containment drywell of heated, pressurized fluid and venting of condensate and non-condensable gasses from the isolation condensers to the suppression pool, is possible because a pressure differential exists between these fluids and gasses on the one hand, and the airspace above the suppression pool water on the other hand.
In other nuclear reactor systems, LOCA heat loads are dissipated in different manner. For example, a type of nuclear system that was built in some numbers in the 1960's and 1970's has a containment which includes an upper space in which the nuclear reactor is disposed, and a lower space defining a suppression pool chamber in which cooling water is present with there being an airspace above the water. The upper and lower spaces are separated by a horizontal structural element, e.g., a concrete floor. A concrete pedestal extends upwardly a distance from the concrete floor in the upper space and serves as a mounting on which the reactor pressure vessel is received and supported. A plurality of vertically disposed vent tubes are arranged in circle array in the floor and have entry ends communicating with the upper space, lower outlet ends of these vent tubes locating submerged in the suppression pool water.
On happening of a LOCA, initial heat is dissipated by heated, pressurized fluid present in the upper space or drywell venting through the vent pipes into the suppression pool wherein steam condenses and non-condensable gasses such as nitrogen cool and vent from the pool water to the airspace above the water. Initial heat also can be dissipated by recirculating water from the reactor vessel to a cooling operation (unless a reactor vessel rupture is present), which cooling operation may for environmental safety reason, involve an intermediate heat exchange location and a final heat exchange location, the latter being one outside the containment. Recirculation of the suppression pool water in like manner can be practiced to take into account that the suppression pool will heat up quite quickly. Decay heat dissipation will be handled by the same suppression pool and reactor vessel water recirculation functions. It is to be noted though that these systems do not employ passive heat removal capacity.
While the last-discussed systems are designed to handle any anticipated LOCA heat load, there is a drawback and potential risk that the cooling function of the suppression pool as it regards non-condensable gasses, can be rendered ineffective. Such happening can come about if a reactor core meltdown attends the LOCA. In that event, the meltdown may cause or contribute to a breaching of the concrete floor structure thereby communicating the drywell of the upper space directly with the airspace above the pool in the lower space rather than such communication being indirect through the suppression pool first. The result is that no lower pressure space exists in the containment to which the higher pressure non-condensable gasses can be vented and cooled by passage through the suppression pool.
The systems with the above-recited shortcoming embody massive containment structures. This works against conveniently and simply making system modifications to counter the effects of meltdown as described above and provide for cooling, both as to initial heat removal and the longer term decay heat dissipation.